The invention relates to a device for visibility measurement having a light emitter for generating light pulses, a light sensor for registering the light scattered back from scatter zones, for example particles, in the light path upon emission of the light pulses and an evaluation circuit for converting output signals of the light sensor into control signals of a visibility display or the like.
A corresponding device is the subject of German (published, unexamined patent application [DOS] No. 2,328,092. In that case, light is emitted in a predetermined direction in the form of pulses from a light emitter. Wherever there are scatter zones in the light path, a greater or lesser proportion of the light is reflected onto a light sensor alongside the light emitter. To make it possible that only backscattered light of a scatter zone within a predetermined distance is taken into account, the light sensor is coupled to a gate circuit in order to evaluate only those output signals of the light sensor which occur at the times between predetermined pulses of the light emitter.
In the case of the device known from German (published, unexamined patent application [DOS]), No. 2,427,623, the intensity of the backscattered headlamp light is evaluated on a motor vehicle. For this purpose, a light sensor on the vehicle is arranged, which can only be impinged by light from a narrow angular range in front of the vehicle.
In the case of a modified device described in the DOS No. 2,427,623, two identical light sensors, which are exposed to daylight or ambient light in an identical way, are arranged. One light sensor additionally lies in the light path of a light source. The output signals of the light sensors are evaluated by subtraction. Due to the uniform impingement of both light sensors with ambient light or daylight, here the effect of ambient light or daylight is suppressed, so that only the additional light of the light source is considered. The intensity of the light acting from the light source on the assigned light sensor depends on whether or not there are scatter zones, which may be caused for example by fog or the like, in the light path. In this way, it is avoided that the ambient brightness has an effect on the measurement of visibility.
An object of the invention is now to create an even better adapted device for visibility measurement, for use in motor vehicles.
This object is achieved in the case of a device of the type specified at the beginning by the fact that both the backscatter from scatter zones, such as fog or the like, on the outside of the windscreen or windshield of a driver's compartment or cab of a motor vehicle and the backscatter by soiling, such as deposit or the like, on the windshield can be evaluated by the virtue of the fact that the light emitter and the light sensor are arranged inside the windshield and the light pulses are directed onto the windshield and through it to the outside.
The invention is based on the realization that the soiling of the windshield also has very strong effect on the respective visibility of the driver. Due to the arrangement according to the invention, soiled windshields are automatically taken into account in the measurement of visibility since the light sensor is also impinged if soiling on the windshields leads to backscattering of the light. Furthermore, viewing hindrances in the area outside the windshield are registered, since the light pulses are also directed through the windshield and are accordingly reflected on the outside of the windshield by fog or the like to the light sensor.
According to a preferred embodiment of the invention, the light emitter is arranged such that the light pulses are directed at the zone covered by the windshield wiper of the windshield. Accordingly, the result of the visibility measurement is corrected after actuation of the windshield wipers or of a windshield washing system combined therewith.
A laser diode and a photodiode may expediently be arranged as light emitter and light sensor. Due to the small dimensions of these components, the possibility exists of integrating the same into the rear view mirror of a motor vehicle, arranged on the inside of the windshield.
The device according to the invention can, in principle, operate with visible light. However, in order to avoid any disturbance to the driver, preferred embodients expediently provide that the light emitter generates light outside the visible wave range, for example infrared light.
It is to be regarded as a particular advantage that, in addition to visibility measurement, the device according to the invention can also be set up for measuring the distance of a fog zone or the like from the vehicle. Consequently, the possibility exists of warning the driver sufficiently long before entering a fog zone or bank.
For this purpose, it is provided that the evaluation circuit processes separately the output signals of the light sensor after emission of a light pulse for a plurality of predeterminable time windows or intervals, the time lapses between the onset of the light pulse and the beginning of the respective time windows or intervals being predeterminable according to various transit times which are required by the light for covering variously long paths which lead from the light emitter to variously remote scatter zones, for example, fog zones, and from there to the light sensor, and that the lengths of time of the light pulse are shorter than the respective transit times.
In the case of this arrangement, account is taken of the fact that the light emitted by the emitter and scattered back to the light sensor at the scatter zones covers variously long light paths depending on the distance of the scatter zone from the light emitter and from the light sensor. So if the light sensor registers the backscattered light intensitY at various time lapses after generation of the light Pulse by the light emitter, this is equivalent to the intensities of light scattered back at various distances being registered. So if there is a fog bank some distance away from the visibility measuring device, while the view in the region in front of the fog bank is good, the light sensor will register virtually no light intensity within the time windows or intervals which are assigned to relatively short light paths and thus to scatter zones at a relatively small distance. On the other hand, a comparatively strong light intensity is registered within the time windows or intervals which are assigned to a longer light path and thus to scatter zones in the region of the fog bank. Accordingly, the distance of a fog bank can be established by checking within which time windows or intervals a relative increase in the light intensity at the light sensor occurs.
The evaluation circuit may, as appropriate, have further inputs for the travelling speed of a vehicle or the like and/or weather data.
Consequently, the possibility is created that the evaluation circuit actuates, as a function of the output signals of the light sensor and/or of signals at the further inputs, a display for the visibility, a display for a recommended travelling speed, a speed limitation for the vehicle, a windshield wiper, a windshield washing system and/or a defroster of the windshield.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.